Various pathologies call for the delivery of therapeutic devices, e.g., valve repair or valve replacement devices, to the left atrium or left ventricle of the heart (i.e., the left side of the heart). In many applications, therapeutic devices are delivered to the left side of the heart by being passed through the vena cava, into the right atrium, and through the interatrial septum. Such delivery calls for apparatus and methods for puncturing the interatrial septum. In many applications, the desired site for puncture lies in the fossa ovalis, a region of the septum containing tissue of lesser thickness than is typical of the rest of the septum.